Iron is crucial for maintaining normal structure and function of virtually all mammalian cells (see, for example, Voest et al., in Ann. Intern. Med. 120:490-499 (1994) and Kontoghiorghes, G. J., in Toxicol. Letters 80:1-18 (1995)) Adult humans contain 3-5 g of iron, mainly in the form of hemoglobin (58%), ferritin/hemosiderin (30%),. myoglobin (9%) and other heme or nonheme enzyme proteins (Harrison and Hoare, in Metals in Biochemistry, Chapman and Hall, New York, 1980).
Total iron levels in the body are regulated mainly through absorption from the intestine and the erythropoietic activity of the bone marrow. Upon absorption, iron is transported to various tissues and organs by the serum protein transferrin. Once transported to the target tissue or organ, iron is transported and stored intracellularly in the form of ferritin/hemosiderin. Under normal conditions, transferrin is about 30% saturated with iron in healthy individuals, and an equilibrium is maintained between the sites of iron absorption, storage and utilization. The presence of these homeostatic controls ensures the maintenance of physiological levels of not only iron, but also other essential metal ions such as copper, zinc and cobalt.
Breakdown of these controls could result in metal imbalance and metal overload, causing iron overloading toxicity and possibly death in many groups of patients, especially those with idiopathic hemochromatosis (see, for example, Guyader et al., in Gastroenterol. 97:737-743 (1989)). Among its toxic effects, iron is known to mediate a repertoire of oxygen related free radical reactions (see, for example, Halliwell and Gutteridge, in Halliwell and Gutteridge, Free Radicals in Biology and Medicine, 2nd edition. Oxford: Clarendon Press, 15-19 (1989)). For example, iron, particularly in the form of free iron ions, can promote the generation of reactive oxygen species through the iron-catalyzed Haber-Weiss reaction (see, for example, Haber and Weiss, in Proc. R. Soc. Ser. A. 147:332 (1934)) as follows: EQU Fe.sup.3+ +.O.sub.2.sup.- .fwdarw.Fe.sup.2+ +O.sub.2 EQU Fe.sup.2+ +H.sub.2 O.sub.2 .fwdarw.Fe.sup.3+ +.OH+OH.sup.-
The net result of these reactions is as follows: EQU .O.sub.2.sup.- +H.sub.2 O.sub.2 .fwdarw..OH+OH.sup.- +O.sub.2.
The Haber-Weiss reaction is seen to produce the hydroxyl radical (.OH), a highly potent oxidant which is capable of causing oxidative damage to lipids, proteins and nucleic acids (see, for example, Lai and Piette, in Biochem. Biophys. Res. Commun. 78:51-59 (1977); and Dizdaroglu and Bergtold, in Anal. Biochem., 156:182 (1986)).
The occurrence of iron imbalance resulting in excessive in vivo iron levels can be categorized into two conditions, namely iron-overload and non-iron overload conditions (see, for example, Voest et al., supra; Kontoghiorghes, supra). Iron-overload conditions are common in such patients as those suffering from thalassemia, sickle cell anemia, repeated blood transfusion and hereditary hemochromatosis. In such patients, transferrin is fully saturated with iron, and excess low-molecular-weight iron appears in the serum. This low-molecular-weight iron appears to originate from the iron released mainly from the liver and spleen, and from the breakdown of effete red cells. Other iron overload diseases and conditions include hereditary spherocytosis, hemodialysis, dietary or latrogenic iron intake, intramuscular iron dextran and hemolytic disease of the newborn (see, for example, Voest et al., supra; Kontoghiorghes, supra).
Non-iron overload conditions relate to situations where elevated iron levels are the result of therapeutic intervention, such as, for example, anthracycline anti-cancer therapy or inflammatory diseases such as rheumatoid arthritis. While anthracyclines such as adriamycin (doxorubicin) are effective in the treatment of a number of neoplastic diseases, these compounds have limited clinical utility due to the high incidence of cardiomyopathy (see, for example, Singal et al., in J. Mol. Cell. Cardiol. 30 19:817-828 (1987)).
The molecular mechanism of cardiomyopathy is now attributed to the adriamycin-induced release of iron from intracellular iron-containing proteins, resulting in the formation of an adriamycin-iron complex, which generates reactive oxygen species causing the scission and condensation of DNA, peroxidation of phospholipid membranes, depletion of cellular reducing equivalents, interference with mitochondrial respiration, and disruption of cell calcium homeostasis (see, for example, Myers et al., Science 197:165-167 (1977); and Gianni et al., in Rev. Biochem. Toxicol. 50:1-82 (1983)). On the other hand, several clinical studies have shown that patients with rheumatoid arthritis exhibit elevated low-molecular weight iron species and ferritin-bound iron levels in synovial fluid. Iron, presumably via its mediation of oxygen free radical pathways, exerts its proinflammatory effects in rheumatoid arthritis (see, for example, Muirden and Senator, in Ann. Rheum. Dis. 27:38-48 (1968); and Biemond et al., in Arthritis Rheum. 29:1187-1193 (1986)).
Iron also plays an important role in many aspects of immune and nonimmune host response (see, for example, De Sousa et al., in Ann. N.Y. Acad. Sci. 526:310-323 (1988)). It is known that increased concentrations of iron are deleterious to the immune system through the initiation or maintenance of inflammatory reactions (see, for example, Biemond et al., in J. Clin. Invest. 73:1576-9 (1984); and Rowley et al., in Clin. Sci. 66:691-5 (1984)). Other non-iron overload diseases and conditions include reperfusion injury, solid tumors (e.g., neuroblastoma), hematologic cancers (e.g., acute myeloid leukemia), malaria, renal failure, Alzheimer's disease, Parkinson's disease, inflammation, heart disease, AIDS, liver disease (e.g., chronic hepatitis C), microbial/parasitic infections, myelofibrosis, drug-induced lung injury (e.g., paraguat), graft-versus-host disease and transplant rejection and preservation.
Hence, not surprisingly, there has been a tremendous interest in the therapeutic use of chelators in the treatment of both iron-overload and non-iron overload diseases and conditions. A chelator (Greek, chele-claw of a crab) is a molecule forming a cyclic ring with a metal as the closing member. Hundreds of chelating agents have been designed and developed for animal and human studies. Among them, at least fifteen different chelators have been used in humans, including desferrioxamine (DF), ethylenediaminetetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA) pyridoxalisonicotinoylhydrazone (PIH), 1,2-dimethyl-3-hydroxypyrid-4-one (L1) and +! 1,2-bis-(3,5-dioxopiperazine-1-yl) propane (ICRF-187).
For the past 30 years, DF (i.e., desferrioxamine) has been the most commonly used chelating drug for the treatment of transfusional iron overload (see, for example, Pippard et al., in Blood 60:288-294 (1982); Proper et al., in N. Engl J. Med. 294:1421-1423 (1976); and St. Louis et al., in Lancet 336:1275-1279 (1990)). Patients suffering from thalassemia lived longer with the DF treatment. However, major drawbacks in the use of DF include the cost thereof (.about.$7,000/patient/year), which can be affordable only by a very small percentage of thalassemia patients worldwide. Another drawback to the use of DF includes the toxicity thereof, including ophthalmic and auditory toxicities as well as induction of pulmonary and renal damage.
Unlike DF, L1 (i.e., 1,2-dimethyl-3-hydroxypyrid-4-one) and related compounds are orally available iron chelators, showing promise in improving the quality of life in patients with thalassemia (see, for example, Olivieri et al., in Drugs Today 28(Suppl. A): 123-132 (1992)) and rheumatoid arthritis (see, for example, Vreugdenhil et al., in Lancet 2:1398-9 (1989)). However, the major side effects of L1 therapy include myelosuppression, fatigue, and maternal, embryo and teratogenic toxicity, which severely limits the potential clinical applications thereof (see, for example, Kontoghiorghes, in Int. J. Hematol. 55:27-38 (1992)).
Recently, ICRF-187 has been demonstrated to be effective in removing iron from the anthracycline-iron complex, therefore preventing the cardiac toxicity in cancer patients receiving adriamycin chemotherapy (see, for example, Kolaric et al., in Oncology 52:251-5 (1995)). However, when chelated with iron, the iron-ICRF-187 complex per se is also very effective in the promotion of hydroxyl radical generation via the Fenton reaction, causing oxidative damage to tissues (see, for example, Thomas et al., in Biochem. Pharmacol. 45:1967-72 (1993)). In addition, since ICRF-187 is a strong chelator (having a structure similar to EDTA), it chelates not only low-molecular-weight iron, but also chelates iron from transferrin and ferritin, as well as copper from ceruloplasmin, thus potentially affecting normal cellular iron metabolism.
Therefore, there is still a need in the art for a new class of iron chelators that are capable of removing free iron ions from body fluids, without affecting the normal cellular iron metabolism.